Uh oh, sounds like green slime algae - this grows real, real fast. I recently had an attack similar to yours., would remove it all and 3 days later, all back again. A plec won't eat this, almost nothing will in a freshwater tank, as it's not really algae, itsa kind of cyano bacteria. Forget chemical treatment as well - not recommendable, or a long term solution. Good news is it's not poisonous in all but a few situations - bad news - it can be a pain to get rid of.
You should test for background phosphate, then I think you have 2 real courses of action. You can go for big time water changes, very frequently, with mechanical removal of the algae (try a fish net). You also need to use a phosphate remover at the same time. This method may, or may not work, as you'll find a little nitrate or phosphate remaining will produce a whole lot of bacteria. I don't think it will for you, reason explained in a minute.
Personally I simply broke the tank down, and rebuilt it. I also put in a lot less gravel this time - I originally had a couple of inches of mixed stuff, under rocks, and I think this was such a nutrient trap, that no matter how often I puled water out, it would release nitrate & phosphate to 'refill' it in a few hours/days. I think you should do the same - if you only have a couple of plants why do you need inches of gravel - it's just hard to keep it clean when it's that deep. If you have a lot of rocks too, that hinders hoovering, and builds the problem. Now my tank has only 1/2 inch average of gravel, still lots of rocks, and only plants like java fern or anubias that I can attach to the rocks, not the substrate.
Note - this only happened in my malawi tank - deep, partly calcaerous gravel, lots of rocks, quite a few plants as well, so hoovering was hard. It doesn't happen too often in tanks with an acid pH, but changing pH is NOT the way forward - way too hard, plus when it all dies your filter will really suffer. Just a note that it never happens in truly soft water either - only hard and alkaline. Also note that I had low water nitrite/nitrate readings when this happened, so it may not be a measure of bad water quality, just a dirty substrate
